The present invention relates to spindle-shaped magnetic iron based alloy particles for high-density recording, which have high output characteristics and a low noise level, and a process for producing the same.
Miniaturization, weight reduction and recording-time prolongation of video or audio magnetic recording and reproducing apparatuses have recently shown a remarkable progress. Especially, regarding video tape recorders (VTR) which have rapidly spread wide, the development of smaller-sized and lighter-weight VTR's for longer-time recording have been rapid. With this progress, magnetic recording media such as a magnetic tape have been strongly required to have a higher performance and a higher recording density.
In other words, magnetic recoding media are required to have higher output and lower the noise level. For this purpose, it is necessary to improve the residual magnetic flux density (Br), the coercive force, the surface smoothness of the magnetic media and the S/N ratio.
These characteristics of magnetic recording media have close relation to the magnetic particles used for the magnetic recording media. In recent years, magnetic iron based alloy particles have attracted attention due to their coercive force and saturation magnetization which are superior to those of conventional magnetic iron oxide particles, and have been put to practical use as magnetic media such as digital audio tapes (DAT), 8-mm video tapes, Hi-8 tapes and video floppies. Such magnetic iron based alloy particles, however, are also strongly demanded to improve the characteristics.
The relationship between various characteristics of magnetic recording media and the properties of magnetic particles used therefor will be described in the following.
In order to obtain a higher recording performance, magnetic recording media for VTR's are required to improve (1) the video S/N ratio, (2) the chroma S/N ratio and (3) the video frequency characteristics, as is obvious from the description in NIKKEI ELECTRONICS, May 3, pp. 82 to 105 (1976).
In order to improve the video S/N ratio, it is important to make the magnetic particles smaller, and to improve the dispersibility of the magnetic particles in a vehicle, the orientation and the loadings of the magnetic particles in a coating film and the surface smoothness of the magnetic recording media.
It is known that a method of lowering the noise level of a magnetic recording medium by reducing the particle size of the magnetic particles used therefore is effective as a method for improving the video S/N ratio.
The particle size of magnetic particles is often expressed by the value of the specific surface area of the particles. It is generally known that the noise level of a magnetic recording medium has a tendency to lower with the increase in the specific surface area of the magnetic particles used.
This phenomenon is shown in, for example, FIG. 1 in Japanese Patent Laid-Open No. 58-159231. The FIG. 1 shows the relationship between the specific surface area of magnetic metal particles and the noise level of the magnetic tape produced therefrom. With the increase in the specific surface area of the particles, the noise level lowers linearly.
Therefore, the magnetic particles are required to have a large specific surface area in order to lower the noise level and improve the video S/N ratio.
However, if the specific surface area of the magnetic particles becomes too large, it becomes more difficult to disperse the magnetic particles in a vehicle (because the amount of binder per unit surface area of the magnetic particles is reduced) and to improve the orientation and the loadings thereof in the coating film, thereby making it impossible to obtain a good surface smoothness, and consequently it leads to the deterioration of the video S/N ratio. Generally, the increase of the specific surface area solely is rather unfavorable. It is therefore important to select the optimum range of a specific surface area in consideration of the technique of dispersing the magnetic particles in a vehicle.
Regarding the relationship between the magnetic metal particles and the noise, it is known that the crystallite size of the magnetic metal particles has a relation to the noise.
This phenomenon is shown in, for example, FIG. 38 on page 123 of the COLLECTED DATA ON MAGNETIC RECORDING MEDIA, Aug. 15 (1985), published by Synthetic Electronics Research. The FIG. 38 shows the relationship between the crystallite size of the magnetic iron based alloy particles and the noise of the magnetic tape produced therefrom. It is observed from FIG. 38 that with the reduction in the crystallite size of the particles, the noise level is lowered.
It is therefore effective for lowering the noise level of a magnetic recording medium to reduce the crystallite size of the magnetic metal particles as much as possible.
As described above, in order to improve the video S/N ratio and lower the noise level, magnetic particles which are excellent in the dispersibility in a vehicle, and the orientation and the loadings in a coating film, have a small crystallite size, an appropriate range (in particular, about 30 to 60 m.sup.2 /g) of a specific surface area and a uniform particle size distribution, and contain no dendrites are required.
In order to improve the chroma S/N, it is necessary to improve the surface property and the squareness ratio of the magnetic recording medium. For this purpose, magnetic particles having good dispersibility and orientation property are useful. Such magnetic particles are required to have a large aspect ratio (major axial diameter/minor axial diameter), an uniform particle size distribution and an appropriate range of a specific surface area, and to contain no dendrites.
Furthermore, in order to improve the video frequency characteristics, it is necessary that the magnetic recording medium has a high coercive force (Hc) and a high remanense (Br).
In order to enhance the remanence (Br) of magnetic media, a high coercive force (Hc) is required. It is important for the magnetic particles used for recording media such as video floppies, DAT's, 8-mm video tapes and Hi-8 tapes to have a coercive force of about 1,300 to 1,700 Oe at present.
Since the coercive force of magnetic particles are generally caused by the shape anisotropy, the coercive force has a tendency to increase with the increase in the aspect ratio (major axial diameter/minor axial diameter). On the other hand, the coercive force has a tendency to reduce with the reduction in the crystallite size. Therefore, if the crystallite size is reduced in order to lower the noise level and to improve the video S/N ratio, the coercive force is lowered and it is difficult to improve the video frequency characteristics. Accordingly, the reduction in the small crystallite size while keeping the coercive force as high as possible is required.
Magnetic particles having a large saturation magnetization (.sigma.s) are necessary for enhancing the residual magnetic flux density (Br) of the magnetic medium, and the residual magnetic flux density (Br) depends upon the dispersibility of the magnetic particles in a vehicle and the orientation and the loadings of the magnetic particles in a coating film.
Although magnetic iron based alloy particles have a larger saturation magnetization than iron oxide magnetic particles, since they are very fine particles having a particle size of not more than 1 .mu.m, the surface activity of the particles is so large that they react with oxygen even they are taken into air after forming an oxide film on the particle surface by the surface oxidation after reduction, resulting in the great deterioration of the magnetic properties, in particular, the saturation magnetization. The deficiency of oxidative stability causes, with time, the deterioration of the saturation magnetic flux density (Bm) and the residual magnetic flux density (Br) of the magnetic recording medium even after the magnetic particles are coated with some binder as a magnetic coating. The saturation magnetization tends to be lowered to a greater extent as the magnetic iron based alloy particles become finer. Therefore, with the recent pronounced tendency to small magnetic particles, a balance in a large saturation magnetization and oxidization stability becomes to be very important. Thus, the method for the surface oxidization of magnetic iron based alloy particles after reduction is an important problem.
Magnetic iron based alloy particles are generally obtained by heat-treating in a reducing gas goethite particles as the starting material, hematite particles obtained by dehydrating the goethite particles at a high temperature, the goethite particles containing metals other than iron or the hematite particles containing metals other than iron.
As a method of producing goethite particles as the starting material, there are known a method(i) of producing acicular goethite particles comprising adding not less than an equivalent of an alkali hydroxide solution to an aqueous ferrous salt solution to obtain a suspension containing ferrous hydroxide particles and carrying out oxidization by passing an oxygen-containing gas into the suspension at a temperature of not higher than 80.degree. C. and at a pH of not less than 11; and a method(ii) of producing spindle-shaped goethite particles comprising reacting an aqueous ferrous salt solution with an aqueous alkali carbonate solution or a mixture of an aqueous alkali carbonate solution and an aqueous alkali hydroxide solution to obtain a suspension containing FeCO.sub.3 or an Fe-containing precipitate and carrying out oxidization by passing an oxygen-containing gas into the suspension.
The acicular goethite particles obtained by the method(i) has a large aspect ratio (major axial diameter/minor axial diameter=not less than 10) but contains dendrites and cannot be said to have an uniform particle size distribution. And the magnetic iron based alloy particles obtained by reduction of these acicular goethite particles have a high coercive force due to the large aspect ratio (major axial diameter/minor axial diameter), but contain dendrites and cannot be said to have an uniform particle size distribution.
On the other hand, the spindle-shaped goethite particles obtained by the method(ii) have an uniform particle size distribution and do not contain any dendrites, but it is difficult to prepare spindle-shaped goethite particles having a large aspect ratio (major axial diameter/minor axial diameter). The preparation becomes more difficult as the particle length produced becomes smaller. The magnetic iron based alloy particles obtained by reduction of these spindle-shaped goethite particles have an uniform particle size distribution and contain no dendrites, so that the dispersibility in a vehicle and the orientation and the loadings thereof in a coating film are excellent, but since the aspect ratio (major axial diameter/minor axial diameter) is small, and as a result it is difficult to obtain particles having a high coercive force.
A method of producing spindle-shaped goethite particles comprising reacting an aqueous alkali carbonate solution with an aqueous ferrous salt solution to obtain a suspension containing FeCO.sub.3 and passing an oxygen-containing gas into the suspension in the presence of a carboxylic acid such as citric acid and tartaric acid and a salt thereof is disclosed in Japanese Patent Application Laid-Open (KOKAI) No. 50-80999. In this case, the goethite particles obtained have a small aspect ratio (major axial diameter/minor axial diameter), as seen from the description of the specification: "Spheroidal particles close to spindle-shaped particles or spherical particles are obtained".
Various attempts have heretofore been conducted to increase the aspect ratio (major axial diameter/minor axial diameter) of spindle-shaped goethite particles so as to obtain magnetic iron based alloy particles which have an uniform particle size distribution without containing any dendrite and a high coercive force. For example, the methods are described in Japanese Patent Application Laid-Open (KOKAI) Nos. 59-232922 (1984), 60-21307 (1985) , 60-21819 (1985) , 60-36603 (1985) , 62-158801 (1987) and 2-51429 (1990). The spindle-shaped magnetic iron based alloy particles which are obtained by these methods, however, disadvantageously have a large crystallite size.
Magnetic iron based alloy particles which have an uniform particle size distribution without containing any dendrite, a high coercive force, a small crystallite size and an appropriate range of specific surface area have been strongly demanded.
As a result of studies undertaken by the present inventors so as to solve these problems, it has been found that spindle-shaped magnetic iron based alloy particles which have a large aspect ratio (major axial diameter/minor axial diameter), an uniform particle size distribution without any dendrite, a small crystallite size, an appropriate range of specific surface area, a high coercive force and a large saturation magnetization can be obtained by aging a suspension containing FeCO.sub.3 or an Fe-containing precipitate, passing an oxygen-containing gas into the aged suspension containing FeCO.sub.3 or an Fe-containing precipitate in the presence of propionic acid or salt thereof to obtain spindle-shaped goethite particles, subjecting the thus-obtained spindle-shaped goethite particles to coating-treatment with some compound, and heat-treating the coated particles in a reducing gas. The present invention has been achieved on the basis of this finding.